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Date: Tue, 6 Feb 2024 20:17:28 -0800
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Subject: Re: [PATCH v12 17/20] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()
From: Sean Christopherson <seanjc@google.com>
To: Paul Durrant <paul@xen.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, David Woodhouse <dwmw2@infradead.org>, Shuah Khan <shuah@kernel.org>, 
	kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, 
	linux-kselftest@vger.kernel.org
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On Mon, Jan 15, 2024, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> As described in [1] compiling with CONFIG_PROVE_RAW_LOCK_NESTING shows that
> kvm_xen_set_evtchn_fast() is blocking on pfncache locks in IRQ context.
> There is only actually blocking with PREEMPT_RT because the locks will
> turned into mutexes. There is no 'raw' version of rwlock_t that can be used
> to avoid that, so use read_trylock() and treat failure to lock the same as
> an invalid cache.

Are rwlocks fundamentally incapable of supporting a raw version?  Because that's
the only argument I see for adding a hack like this.